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Abstract:

A compensation method for a light emitting diode (LED) circuit including
a first transistor, a second transistor, a capacitor, and a LED is
illustrated. A first end as a control end of the first transistor is
connected to one end of the second transistor and the capacitor, and a
second end of the first transistor is connected to the LED. A width to
length (W/L) ratio of the second transistor is less than one. An initial
control voltage is applied to a control end of the second transistor, and
the current output voltage of the LED is correspondingly measured. If a
difference between the current output voltage and an initial output
voltage exceeds a predetermined value, a compensation voltage, which is a
summation of the initial control voltage and the difference, is applied
to the control end of the second transistor.

Claims:

1. A compensation method for a light emitting diode circuit having a
first transistor with a first end as a control end thereof and a second
end and a second transistor having a first end as a control end thereof
and a second end, a capacitor, and a light emitting diode, wherein the
first end of the first transistor is connected to the capacitor and the
second end of the second transistor, the second end of the first
transistor is connected to the light emitting diode, and a width to
length ratio of the second transistor is less than one, the compensation
method comprising: applying an initial control voltage to a control end
of the second transistor, and detecting a current value of an output
voltage of the light emitting diode; determining whether a difference of
the current value of the output voltage of the light emitting diode and
an initial value of the output voltage of the light emitting diode
exceeds a predetermined value; and generating a compensation voltage and
applying the compensation voltage to the control end of the second
transistor when the difference exceeds the predetermined value, wherein
the compensation voltage is summation of the initial control voltage and
the difference.

2. The method according to claim 1 further comprising: applying the
initial control voltage to the control end of the second transistor
continuously if the difference does not exceed the predetermined value.

3. The method according to claim 2 further comprising: re-determining
whether a difference of the current value of the output voltage of the
light emitting diode and the initial value of the output voltage of the
light emitting diode exceeds the predetermined value, wherein the current
value of the output voltage is generated by the light emitting diode
circuit according to the compensation voltage; applying the compensation
voltage continuously when the difference does not exceed the standard
value; and re-generating the compensation voltage when the difference of
the current value of the output voltage and the initial value of the
output voltage exceeds the predetermined value.

4. The method according to claim 1, wherein the method compensates the
light emitting diode circuits of a red pixel, a green pixel, and a blue
pixel, or compensates the light emitting diode circuit of a pixel that
has been degraded the most.

5. An compensation apparatus for a light emitting diode circuit having a
first transistor with a first end as a control end thereof and a second
end, and a second transistor having a first end as a control end thereof
and a second end, a capacitor, and a light emitting diode, wherein the
control end of the first transistors is connected to the capacitor and
the second end of the second transistor, the second end of the first
transistor is connected to the light emitting diode, and a width to
length ratio is less than one, the compensation apparatus comprising: a
plurality of memory units, for memorizing an initial value of an output
voltage of the light emitting diode and a current value of the output
voltage of the light emitting diode, wherein the apparatus applies an
initial control voltage to the control end of the second transistor, such
that the light emitting diode generates the current value of the output
voltage; a logic and comparison unit, for determining whether a
difference of the current value and the initial value of the output
voltage exceeds a predetermined value; and a processing circuit unit,
generating a compensation voltage and applying the compensation voltage
to the control end of the second transistor according to the difference
and the initial control voltage when the difference exceeds the
predetermined value.

6. The compensation apparatus for light emitting diode circuit according
to claim 5, wherein the compensation voltage is summation of the initial
control voltage and the difference.

7. The compensation apparatus for light emitting diode circuit according
to claim 5, wherein the compensation apparatus applies the initial
control voltage to the control end of the second transistor continuously
when the difference does not exceed the predetermined value.

8. The compensation apparatus for light emitting diode circuit according
to claim 5 further comprises another memory unit for memorizing the
compensation voltage, the logic and comparison unit re-determines whether
the difference of the initial value of the output voltage and the current
value of the output voltage exceeds the predetermined value, wherein the
current value of the output voltage is generated by the light emitting
diode according to the compensation voltage, the compensation apparatus
continuously applies the compensation voltage if the difference does not
exceed the standard value, and the compensation apparatus re-generates
the compensation voltage when the difference exceeds the predetermined
value.

9. The compensation apparatus for light emitting diode circuit according
to claim 5, wherein the compensation apparatus compensates the light
emitting diode circuits of a red pixel, a green pixel, and a blue pixel,
or compensates the light emitting diode circuit of a pixel that has been
degraded the most.

10. The compensation apparatus for light emitting diode circuit according
to claim 5, wherein the compensation apparatus and the light emitting
diode are fabricated on a printed circuit board and assemble (PCBA).

Description:

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a light emitting diode circuit; in
particular, to a compensation method and a compensation apparatus for a
light emitting diode circuit having the light emitting diode.

[0003] 2. Description of Related Art

[0004] As development of the semiconductor industry, light emitting diodes
(LEDs) are not only widely utilized for lighting to replace traditional
light bulbs which has low energy efficiency but also applied to the field
of display. After thin film transistor liquid crystal displays (TFT-LCDs)
with backlight module virtually replace cathode ray tube (CRT) displays
organic-LED (OLED)-based display devices were further introduced as they
are associated with characteristics of flexibility, higher efficiency,
higher image contrast, and faster response time, all of which may propel
the OLED-based display devices to be one mainstream displays in the
future.

[0005] Generally, an active-matrix organic-LED is driven by a driving
circuit with two transistors and one capacitor (2T1C). Please refer to
FIG. 1. FIG. 1 shows a circuit diagram of a traditional organic-LED
circuit. The traditional organic-LED circuit 1 comprises a first
transistor T1, a second transistor T2, and a capacitor CS. A first end of
the first transistor T1 is coupled to a high voltage level VDD. A second
end of the first transistor T1 is coupled to an input end of the
organic-LED OLED. An output end of the organic-LED OLED is coupled to a
grounding end. A second end of the second transistor T2 is coupled to a
control end of the first transistor T1. A first end of the second
transistor T2 receives a pixel signal DATA. The control end of the second
transistor T2 receives a scanning signal SCAN. The capacitor CS has a
first end coupled to the control end of the first transistor T1 and the
second end of the second transistor T2. A second end of the capacitor CS
is coupled to the grounding end.

[0006] The organic-LED OLED emits light according to an output current
IOLED generated by a driving voltage VDATA of the first transistor T1.
The magnitude of the light is controlled by the pixel signal DATA and the
scanning signal SCAN. After the first transistor T1 has been in operation
for an extended period of time, stress of the gate to source voltage Vgs
(or increase in voltage level of the same) would cause a threshold
voltage Vth of the first transistor T1 to drift to a higher voltage
level. Therefore, the output current IOLED and the output voltage VOLED
of the organic-LED would decrease, negatively affecting light-emitting
efficiency of the OLED and reducing lifespan thereof.

[0007] FIG. 2A and 2B show curves of simulated output current of the
traditional organic-LED circuit with respect to the threshold voltage of
the first transistor. FIG. 2A is represented in terms of a micro-ampere
(μA) with the output current in FIG. 2B being normalized. As shown in
FIG. 2A and FIG. 2B, if the threshold voltage Vth of the first transistor
T1 increases from 0V to 3V, the output current IOLED of the organic-LED
OLED may decrease from 2.05 micro-amperes to 1.3 micro-amperes.

[0008] Since the output current of the organic-LED would decrease to
undermine the light-emitting efficiency of the organic-LED, the quality
of lighting instruments or display devices which utilize the traditional
LED circuits deteriorate or degrade as the result.

SUMMARY OF THE INVENTION

[0009] The object of the present invention is to provide a compensation
method and apparatus for a light emitting diode circuit.

[0010] In order to achieve the aforementioned objects, according to an
embodiment of the present invention, the compensation method is utilized
for at least one light emitting diode which includes a first transistor,
a second transistor, a capacitor, and a light emitting diode. A control
end of the first transistor is connected to the capacitor and a second
end of the second transistor. A second end of the first transistor is
connected to the light emitting diode, and a width to length ratio (W/L)
of the second transistor is less than one. The compensation method
includes applying an initial control voltage to a control end of the
second transistor, and detecting a current value of an output voltage of
the light emitting diode, determining whether a difference of a current
value and an initial value of the output voltage exceeds a predetermined
value, and generating a compensation voltage and applying the
compensation voltage to the control end of the second transistor if the
difference exceeds the predetermined value. The compensation voltage is
summation of the initial control voltage and the difference.

[0011] In order to achieve the aforementioned objects, according to an
embodiment of the present invention, a compensation apparatus for light
emitting diode circuit is utilized for at least one light emitting diode
having a first transistor, a second transistor, a capacitor, and a light
emitting diode. A control end of the first transistor is connected to the
capacitor and a second end of the second transistor. A second end of the
first transistor is connected to the light emitting diode, and the width
to length (W/L) ratio of the second transistor is less than one. The
compensation apparatus may further include a plurality of memory units, a
logic and comparison unit, and a processing circuit unit. The memory
units are used for memorizing an initial value and a current value of the
output voltage of the light emitting diode. The compensation apparatus is
configured to apply an initial control voltage to a control end of the
second transistor, such that the light emitting diode generates the
current value of the output voltage accordingly. The logic and comparison
unit is used to determine whether a difference of the current value and
the initial value of the output voltage exceeds a predetermined value.
The processing circuit unit generates a compensation voltage and applies
the compensation voltage to the control end of the second transistor
according to the difference and the initial control voltage if the
difference exceeds the predetermined value.

[0012] In summary, the method and apparatus for light emitting diode
circuit detect the output voltage of the light emitting diode, and
stabilizes the output current of the light emitting diode by maintaining
the output voltage of the light emitting diode. Therefore, potential
decrease in the output current of the light emitting diode as the result
of operating for the extended period of time may be avoided, extending
the lifespan of the light emitting diode.

[0013] In order to further the understanding regarding the present
invention, the following embodiments are provided along with
illustrations to facilitate the disclosure of the present invention.

[0015] FIG. 2A and 2B show curves simulated output current of the
traditional organic-LED circuit with respect to the threshold voltage of
the first transistor;

[0016]FIG. 3 shows a flow diagram of a compensation method for light
emitting diode circuit according to an embodiment of the present
invention;

[0017]FIG. 4 shows a circuit diagram of a compensation apparatus for
light emitting diode circuit according to an embodiment of the present
invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] The aforementioned illustrations and following detailed
descriptions are exemplary for the purpose of further explaining the
scope of the present invention. Other objectives and advantages related
to the present invention will be illustrated in the subsequent
descriptions and appended drawings.

[0020] Please refer to FIG. 1 and FIG. 3. FIG. 3 shows a flow diagram of a
compensation method for a light emitting diode circuit according to an
embodiment of the present invention. The compensation method for light
emitting diode circuit is utilized for compensating an organic-LED
circuit 1 having transistors T1 and T2, a capacitor CS, and an
organic-LED OLED. A first end serving as a control end of the first
transistor T1 is connected to the capacitor CS and a second end of the
second transistor T2. In one implementation, the control end is a gate
end of the first transistor T1 while the second end of the second
transistor T2 is a source end thereof. A second end of the first
transistor T1, which may be a source end of the first transistor T1, is
connected to an input end of the organic-LED OLED. A width to length
(W/L) ratio of the second transistor T2 in one implementation may be less
than one. It is worth noting that the compensation method for light
emitting diode circuit disclosed in the present invention may be applied
to other light emitting diode circuits that are 2T1C (two transistors and
one capacitor) in their structure.

[0021] The compensation method illustrated in FIG. 3 may increase a
voltage of a first end, which serves as a control end, of the second
transistor T2 to stabilize an output voltage VOLED of the organic-LED
OLED. In one implementation, the control end of the second transistor T2
is the gate end of the second transistor T2. The voltage applied to the
control end of the second transistor T2 needs not to be a high voltage
(for example, 5 Volts), such that the voltage of the control end of the
first transistor T1 may not be fully charged to a voltage VDATA of a
pixel signal (DATA) during a charging period.

[0022] When the voltage of the control end of the second transistor T2
increases, the voltage of the control end of the first transistor T1
increases accordingly. The voltage of the control end of the first
transistor T1 may thus approach to the voltage VDATA of the pixel signal
(DATA) during the charging period so as to compensate the decrease in the
output voltage VOLED due to the drift of a threshold voltage Vth of the
first transistor T1. As such, an output current IOLED may be stabilized

[0023] It is worth noting that the compensation method illustrated in FIG.
3 may be performed periodically (for example, per 1000 hours). In other
words, the decrease in the output current IOLED as the result of the
increase in the voltage at the gate end of the first transistor T1 may be
compensated for every predetermined period.

[0024] Please refer to FIG. 1 and FIG. 3 again. First, in accordance with
step S31, the compensation method of the present invention includes
applying an initial control voltage Vgate_ini to the control end of the
second transistor T2 (in other words, setting a scanning SCAN as the
initial control voltage), and detecting a current value VOLED_now of the
output voltage VOLED of the organic-LED OLED. In step S32, the
compensation method further includes determining whether a difference of
the current value VOLED_now of the output voltage VOLED and an initial
value VOLED_ini of the output voltage VOLED exceeds a predetermined
standard value. In one implementation, the predetermined value is zero.
When the difference does not exceed the predetermined value, the
compensation method goes to step S34; otherwise, the compensation method
performs step S33.

[0025] It is worth mentioning that the initial control voltage Vgate_ini
is an initial driving voltage of the organic-LED circuit 1. The initial
value VOLED_ini of the output voltage VOLED is an initial output voltage
of the organic-LED circuit 1 at the time the initial control voltage
Vgate_ini is applied to drive the organic-LED circuit 1. As the initial
values of the initial driving voltage and the output voltage of the
organic-LED circuit 1 may be necessary for the determination of whether
the differences between them and their counterparts after the organic-LED
circuit 1 has been in operation, the compensation method in FIG. 3 may
further include memorizing the initial control voltage Vgate_ini and the
initial value VOLED_ini of the output voltage VOLED in a plurality of
memory units. In one implementation, the memory unit may be a register.

[0026] In accordance with step S34, if the difference does not exceed the
predetermined value, which is indicative of the output current IOLED does
not decrease due to continuing operations, the compensation method
therefore apples the initial control voltage Vgate_ini to the control end
of the second transistor T2 continuously. In accordance with step S33, if
the difference, however, exceeds the predetermined value, which indicates
that the output current IOLED has decreased due to the long-term
operations, the compensation method further includes generating a
compensation voltage Vgate and applying the compensation voltage Vgate to
the control end of the second transistor T2. In other words, the
compensation method of the present invention sets the applied
compensation voltage Vgate as the scanning signal SCAN in FIG. 1). In one
implementation, the compensation voltage Vgate is a summation of the
initial control voltage Vgate_ini and the difference, which is equal to
Vgate_ini+VOLED_ini-VOLED_now.

[0027] In step S32, the compensation method of the present invention may
utilize a comparator to determine whether the difference of the current
value VOLED_now and the initial value VOLED_ini of the output voltage
VOLED exceeds the predetermined value. In step S33, the compensation
method may employ an invert amplifier to calculate the difference of the
initial value VOLED_ini and the current value VOLED_now of the output
voltage VOLED according to the initial value VOLED_ini and the current
value VOLED_now of the output voltage VOLED (VOLED_ini-VOLED_now). With
the invert amplifier, an inverted difference (-VOLED_ini+VOLED_now) may
be prepared. Thereafter, the compensation method further relies on the
invert amplifier to calculate the compensation voltage Vgate according to
the difference and the initial control voltage Vgate_ini.

[0028] In step S35, the compensation method further includes
re-determining whether the difference of the current value VOLED_now of
the output voltage VOLED and the initial value VOLED_ini of the output
voltage VOLED exceeds the predetermined value. In step S35, the current
value VOLED_now of the output voltage VOLED is generated by the light
emitting diode circuit 1 according to the compensation voltage Vgate.
Then, if the difference does not exceed the predetermined value, step S36
may be performed. Otherwise, the compensation method of the present
invention returns to step S33. In step S36, the compensation method
applies the compensation voltage Vgate to the control end of the second
transistor T2 continuously. In doing so, the compensation method of the
present invention may ensure the difference of the initial value
VOLED_ini and the current value VOLED_now of the output voltage VOLED
does not exceed the predetermined value by repeating steps S33˜S36.

[0029] In practical applications, light emitting diode may be utilized in
color displays, and each pixel of the color display may correspond to the
light emitting diodes of a red pixel, a green pixel, and a blue pixel.
The compensation method according to the embodiment of the present
invention may compensate the light emitting diodes of the red pixel, the
green pixel, and the blue pixel, or direct to the light emitting diode of
a pixel that has been degraded the most.

[0031] Please refer to FIG. 4. FIG. 4 shows a circuit diagram of a
compensation apparatus 4 for a light emitting diode circuit according to
an embodiment of the present invention. The compensation apparatus 4 is
utilized for a light emitting diode circuit. In one implementation, the
light emitting diode circuit is an organic light emitting diode circuit
7. The light emitting diode circuit 7 may include a first transistor T1,
a second transistor T2, a capacitor CS, and an organic-LED OLED. Both the
first transistor T1 and the second transistor T2 may include first ends
and second ends, respectively. The first end as a control end of the
first transistor T1 is connected to the capacitor CS and the second end
of the second transistor T2. The second end of the first transistor T1 is
connected to an input end of the organic-LED OLED. The first end of the
first transistor T1 is coupled to a voltage level VDD. An output end of
the organic-LED OLED is coupled to another voltage level VSS. It is worth
noting that the voltage level VDD may be higher than the voltage level
VSS. The first end of the second transistor T2 is coupled to the pixel
signal VDATA. The control end (or the first end) of the second transistor
T2 is coupled to an initial control voltage Vgate_ini or a compensation
voltage Vgate. The width-to-length ratio (W/L) of the second transistor
T2 may be less than one. It is worth mentioning that the organic-LED OLED
is a transistor having a first end and a control end, which may be a gate
end of the transistor, coupled to each other.

[0032] The compensation apparatus 4 for light emitting diode circuit may
further include a first memory unit 42, a second memory unit 43, a logic
and comparison unit 44, a processing circuit unit 45, a switching device
46, and switches 47 and 48. The processing circuit unit 45 may further
include a first differential amplifier 451, a second differential
amplifier 452, and a third differential amplifier 453.

[0033] The first memory unit 42 and the second memory unit 43 are coupled
to the input end of the organic-LED OLED through switches 47 and 48,
respectively. The logic and comparison unit 44 is coupled to the first
memory unit 42 and the second memory unit 43. An input end of the
processing circuit unit 45 is coupled to the logic and comparison unit
44. An output end of the processing circuit unit 45 is coupled to the
control end of the second transistor T2. Besides, another memorizing unit
(not shown in FIG. 4) may be coupled to the output end of the processing
circuit 45 for memorizing the compensation voltage Vgate.

[0034] The first memory unit 42 and the second memory unit 43 may be
configured to memorize the current value VOLED_now and the initial value
VOLED_ini of the output voltage VOLED of the organic-LED OLED,
respectively. In one implementation, the switch 47 may only be turned on
when the organic light emitting diode circuit 7 is just driven. On the
other hand, the switch 48 may be turned on for a predetermined period of
time.

[0035] When the light emitting diode is just driven, the switching device
46 may switch to the initial control voltage Vgate_ini with the switch 47
turned on, thus the first memory unit 42 memory the initial value
VOLED_ini of the output voltage VOLED generated by the organic light
emitting diode circuit 7 according to the initial control voltage
Vgate_ini. The compensation apparatus 4, thereafter, may apply the
initial control voltage Vgate_ini to the control end of the second
transistor T2 through the switching device 46, so that the organic-LED
OLED may generate the current value VOLED_now of the output voltage
VOLED, which may be stored in the second memory unit 43.

[0036] Meanwhile, the logic and comparison unit 44 compares the initial
value VOLED_ini and the current value VOLED_now of the output voltage
VOLED of the organic-LED OLED, before the difference VO may be obtained
and whether the difference VO exceeds the predetermined value may be
determined. By determining the difference VO on basis of the different
output voltages VOLED (i.e., the different current values VOLED_now), the
logic and comparison unit 44 may cause the processing circuit unit 45 to
generate the compensation voltage Vgate. More specifically, when the
organic light emitting diode circuit 7 operates for a predetermined
period of time, the logic and comparison unit 44 of the compensation
apparatus 4 may be enabled for determining whether the difference VO
exceeds the predetermined value before the processing circuit 45 may be
further enabled.

[0037] If the difference VO does not exceed the predetermined value, which
may indicate that there is no need to change the voltage applied to the
control end of the second transistor T2 as the compensation apparatus 4
may control operation of the switching device 46 so as to continuously
output the initial control voltage Vgate_ini to the control end of the
second transistor T2. If the difference VO exceeds the predetermined
value, the compensation apparatus 4 may cause the compensation voltage
Vgate to be applied to the control end of the second transistor T2.

[0038] In one implementation, gains of the first differential amplifier
451, the second differential amplifier 452, and the third differential
amplifier 453 are configured to be one. When the difference VO exceeds
the predetermined value, the logic and comparison unit 44 may output the
initial value VOLED_ini of the output voltage VOLED of the organic-LED
OLED to a positive input end of the first differential amplifier 451 of
the processing circuit unit 45. And, the logic and comparison unit 44 may
output the current value VOLED_now of the output voltage VOLED of the
organic-LED OLED to a negative input end of the first differential
amplifier 451 of the processing circuit unit 45, leading to the first
differential amplifier 451 to output the difference VO of the initial
value VOLED ini and the current value VOLED_now of the output voltage
VOLED.

[0039] A positive input end of the second differential amplifier 452 may
be grounded. A negative input end of the second difference is coupled to
the difference VO, such that the second differential amplifier 452
outputs a negative difference VO. Meanwhile, a positive input end of the
third differential amplifier is coupled to the initial control voltage
Vgate_ini, and a negative input end of the third differential amplifier
453 is coupled to the negative difference VO. Under this arrangement, the
third differential amplifier 453 may output the compensation voltage
Vgate, which is equal to Vgate_ini+VO.

[0040] Further more, in practical applications, light emitting diodes can
be utilized to the color displays with light emitting diodes of three
primary colors. Accordingly, the compensation apparatus 4 may be
configured to compensate the light emitting diodes of the three primary
colors, or compensate the light emitting diode that has been degraded the
most.

[0041] Besides, the compensation apparatus 4 may be fabricated on a
printed circuit board and assemble (Printed Circuit Board and Assemble,
PCBA) for simplifying fabrication process.

[0042] In summary, the mentioned compensation apparatus for light emitting
diode circuit can detect or detect periodically output voltage of light
emitting diodes. The compensation apparatus for light emitting diode
circuit maintains output voltage of light emitting diodes for stabilizing
operation current of light emitting diodes. Accordingly, deterioration of
operation current of the light emitting diode due to long-term operation
can be avoided.

[0043] The descriptions illustrated supra set forth simply the preferred
embodiments of the present invention; however, the characteristics of the
present invention are by no means restricted thereto. All changes,
alternations, or modifications conveniently considered by those skilled
in the art are deemed to be encompassed within the scope of the present
invention delineated by the following claims.

Patent applications by Chin-Wen Lin, Hsinchu City TW

Patent applications by Ted-Hong Shinn, Hsinchu City TW

Patent applications by E Ink Holdings Inc.

Patent applications in class CURRENT AND/OR VOLTAGE REGULATION

Patent applications in all subclasses CURRENT AND/OR VOLTAGE REGULATION